This invention relates to electric discharge lasers and in particular to such lasers having chambers with long life electrodes.
The principal components of a prior art KrF excimer laser system are shown in FIGS. 1, 2 and 3. The laser system is used as a light source for integrated circuit lithography. These components include a laser chamber housing 2. The housing contains two electrodes 84 and 83 each about 50 cm long and spaced apart by about 20 mm, a blower 4 for circulating a laser gas between the electrodes at velocities fast enough to clear (from a discharge region between the two electrodes) debris from one pulse prior to the next succeeding pulse at a pulse repetition rate in the range of 1000 Hz to 4,000 Hz or greater, and one or more water cooled finned heat exchanger 6 for removing heat added to the laser gas by the fan and by electric discharges between the electrodes. The word xe2x80x9cdebrisxe2x80x9d is used here to define any physical condition of the gas after a laser pulse, which is different from the condition of the gas prior to the pulse. The chamber may also include baffles and vanes for improving the aerodynamic geometry of the chamber. The laser gas is comprised of a mixture of about 0.1 percent fluorine, about 1.0 percent krypton and the rest neon. Each pulse is produced by applying a very high voltage potential across the electrodes with a pulse power system 8, shown as an electrical circuit in FIG. 3, which causes discharges (between the electrodes) lasting about 30 nanoseconds to produce a gain region about 20 mm high, 3 mm wide and 500 mm long. Each discharge deposits about 2.5 J of energy into the gain region. As shown in FIG. 2, lasing is produced in a resonant cavity, defined by an output coupler 2A and a grating based line narrowing unit (called a line narrowing package or LNP, shown disproportionately large) 2B comprising a three prism beam expander, a tuning mirror and a grating disposed in a Littrow configuration. The energy of the output pulse 3 in this prior art KrF lithography laser is typically about 10 mJ.
This KrF laser light source produces a narrow band pulsed ultraviolet light beam with a wavelength at about 248 nm. These lasers typically operate in a so-called xe2x80x9cburst modexe2x80x9d consisting of bursts of pulses at a pulse repetition rate in the range of about 1000 to 4000 Hz. Each burst consists of a number of pulses, for example, about 80 to 300 pulses, each burst illuminating a single die section on a wafer with the bursts separated by off times of a fraction of a second while the lithography machine shifts the illuminating beam between die sections. There is another longer off time of a few seconds when a new wafer is loaded. Therefore, in production, for example, a 2000 Hz, KrF excimer laser may operate at a duty factor of about 30 percent. The operation is 24 hours per day, seven days per week, 52 weeks per year. A laser operating at 2000 Hz xe2x80x9caround the clockxe2x80x9d at a 30 percent duty factor will accumulate more than 1.5 billion pulses per month. Any disruption of production can be extremely expensive. For these reasons, prior art excimer lasers designed for the lithography industry are modular. The modules typically can be replaced within a few minutes so that maintenance down time is minimized. Laser availability of these lasers is typically higher than 99 percent.
Maintaining high quality of the laser beam produced by these lasers is very important because the lithography systems in which these laser light sources are used are currently required to produce integrated circuits with features smaller than 0.25 microns and feature sizes get smaller each year. As a result the specifications placed on the laser beam limit the variation in individual pulse energy, the variation of the integrated energy of series of pulses, the variation of the laser wavelength and the magnitude of the spectral bandwidth of the laser beam.
Prior art electrodes for the gas discharge lasers referred to above are typically about 50 cm long, may be about 3 cm wide and may have cross section shapes similar to those shown in FIG. 1 at 83 and 84. The actual discharges between the electrodes typically need to be a few millimeters wide (e.g., 3-4 mm) and this need determines the shape of the electrodes. The two electrodes shown produce relatively very high electrode fields over a 3-4 mm wide region in the central region of both of the electrodes (called herein the discharge footprint or discharge surface) to produce approximately rectangles discharges about 3-4 mm in width with a height approximately equal to the electrode spacing and the length of the discharge region is about 500 cm. One problem with these prior art electrodes is that erosion in the approximately 3-4 mm discharge footprint port of both electrodes over several billion pulses causes changes in the cross section shape of the electrode which alters the electric fields which in turn affect the discharge footprint so that the discharge shape is no longer uniform and may become substantially wider, narrower, split or otherwise distorted thereby adversely affecting laser beam quality, and reducing laser efficiency.
Electrode designs have been proposed which are intended to minimize the effects of erosion by providing on the electrode a protruding part having the same width as the discharge. Some examples are described in Japanese Patent No. 2631607. These designs, however, produce adverse effects on gas flow if the protrusion is large and if the protrusion is small; it is eroded away relatively quickly.
Other gas discharge lasers used as lithography light sources, very similar to the KrF laser, are the ArF (argon fluorine) laser and the F2 (fluorine molecular laser). In the ArF laser the active gases are a mixture primarily of argon and fluorine with neon as a buffer gas, and the wavelength of the output beam is in the range of about 193 nm. These ArF lasers are just now being used to a significant extent for integrated circuit fabrication, but the use of these lasers is expected to grow rapidly. In the F2 laser, expected to be used extensively in the future for integrated circuit fabrication, the active gas is F2 and a buffer gas could be neon or helium or a combination of neon and helium. All of these gas discharge lithography lasers utilize similar electrodes although the spacing between them may be slightly different.
What is needed is a gas discharge laser having electrodes which do not adversely affect gas flow and can withstand many billions of electric discharges without substantial adverse effects on laser beam quality.
The present invention provides a gas discharge laser having at least one long-life elongated electrode for producing at least 12 billion high voltage electric discharges in fluorine containing laser gas. In a preferred embodiment at least one of the electrodes is comprised of a first material having a relatively low anode erosion rate and a second anode material having a relatively higher anode erosion rate. The first anode material is positioned at a desired anode discharge region of the electrode. The second anode material is located adjacent to the first anode material along at least two long sides of the first material. During operation of the laser erosion occurs on both materials but the higher erosion rate of the second material assures that any tendency of the discharge to spread onto the second material will quickly erode away the second material enough to stop the spread of the discharge. In a preferred embodiment the anode is as described above and the cathode is also a two-material electrode with the first material at the discharge region being C26000 brass and the second material being C36000 brass. A pulse power system provides electrical pulses at rates of at least 1 KHz. A blower circulates laser gas between the electrodes at speeds of at least 5 m/s and a heat exchanger is provided to remove heat produced by the blower and the discharges.
In preferred embodiments the two-material electrode is an anode of a fluorine containing gas discharge laser. A portion of the anode located at the discharge surface of the anode, is comprised of an anode material containing lead along with other metals chosen to produce during operation a porous insulating layer covering the discharge surface of the anode. The layer is produced by fluorine ion sputtering of the anode surface which creates the insulating layer comprised in part of lead fluoride as well as fluorides of other metals. In a particular preferred embodiment the anode is fabricated in two parts, a second part having the general shape of a prior art anode with a trench shaped cavity at the top. The material for this part such as C26000 brass will be eroded if subject to electric discharge in the normal discharge laser gas environment. A first part comprised of brass having a lead content of greater than 3% is soldered into the trench and protrudes above the surface by about 0.2 millimeter. When the anode is installed in the laser and is subjected to pulse discharges in a fluorine containing laser gas environment an insulating layer, comprising porous lead fluoride, forms on the surface of the first part protecting it from significant erosion. Applicants"" computer electric field models have shown that the insulating layer does not significantly affect the electric field between the cathode and the anode. The overall electrode shape is such that there are no significant discharges from the second part at beginning of operation with the electrodes. To the extent discharges do occur from the second part, erosion will occur at the discharge sites reducing the height of the anode in the region of the discharge which has the effect of reducing the discharge from the second part. About 50,000 small holes develop in the insulating layer on the first part which permit electrons to flow freely to and from the metal surface of the anode. However, fluorine ion sputtering on the metal surface of the anode is substantially limited after the insulating layer has developed. Applicants believe that the reduction in fluorine ion sputtering results from a reduced number of fluorine ions reaching the metal surface and a reduction in energy of the ions that do reach the metal surface.
Applicants"" tests have shown that the porous insulating layer that covers substantially all of the discharge surface of the anode does not significantly interfere with the electric field between the electrodes and helps control the shape of the discharge making it more spatially uniform over chamber life, as compared to prior art anode designs. This increase uniformity in discharge shape results in greatly improved laser pulse quality over chamber life. Better discharge shape also minimizes the adverse effect of acoustic disturbances within the chamber resulting from reflected acoustic waves from one pulse reflecting back into the discharge region during the immediately following pulse.
Embodiments of the present invention provide decreased burn-in times extended operating lifetimes and improved laser beam quality and beam stability.